If he was added, how would you want to see them handle djinn/summoning? Isaac is tricky to make a moveset for since he has *so* many options, but that's a whole layer entirely.
I think a summon for a final smash would be easiest and the most appropriate. Djinn I'm not so sure, and it wouldn't surprise me if they didn't incorporate djinn into Isaac's moveset for simplicity.
If I were to incorporate Djinn without being too complicated, I would give Isaac additional effects to smash attacks that are sufficiently charged with some extra animations of Djinn hovering around Isaac during the attacks.
For example;
Side Smash: Isaac slashes with his sword. If he charges long enough, Flint appears and causes bonus damage.
Down Smash: Isaac summons two rocks on both sides (just like Smash Flash 2). If he charges long enough Granite appears and grants Isaac bonus resistance.
Up Smash: Isaac summons a large thorny vine reaching reasonably high above him. If Isaac charges long enough, the Djinn Ground pulls the opponent down and into the ground, buried.
Balancing-wise I'd make it so that Isaac can't get any boosted smash attack effects for a short period of time. I'm sure we'd all love to see Isaac use different summons depending on how many Djinn he has on standby but I think we can all agree that's probably unlikely to happen which is fine.